PROJECT SUMMARY Our eyes are never at rest, even when attending to a single point. We are normally not aware that, in the peri- ods of fixation, microscopic eye movements continually shift the stimulus on the retina. Visual percepts tend to fade when the stimulus is artificially immobilized on the retina, and it has long been hypothesized that the inces- sant fixational motion of the eye plays a fundamental role in visual perception. Several findings from our recent NIH-funded research suggest that this motion is, in fact, a critical computational element of an active sensori- motor strategy by which the visual system processes spatial information in the temporal domain. Building upon our recent results, this project examines the perceptual, computational, and neural consequences of using eye movements to represent space through time. It addresses three fundamental questions: (Aim 1) How is spatial information encoded in the modulations of luminance resulting from fixational eye movements? (Aim 2) How is this information extracted and interpreted? (Aim 3) Can the spatiotemporal redistribution of input energy be adjusted according to task by controlling fixational eye movements? To link the perceptual influences of fixational eye movements to their effect on the neural coding of visual information and elucidate encoding/decoding mech- anisms, this project integrates visual psychophysics in humans, statistical and computational analysis of retinal input, and neural modeling. Experiments will make critical use of a sophisticated system for gaze-contingent display (already developed and extensively tested), which allows precise, yet highly flexible control of retinal stim- ulation. Statistical, computational, and modeling studies will reconstruct and examine the visual input signals experienced by retinal receptors and simulate neuronal responses in the retina and lateral geniculate nucleus. The proposal that the visual system uses behavior to represent space through time challenges current views on the mechanisms of early visual processing at the most fundamental level: it replaces the traditional notion of the retina as a passive encoding stage that optimizes overall information transmission with that of an active, tunable system for feature extraction, whose function can only be understood in conjunction with eye movements. This shift of view implies that eye movements are in part responsible for fundamental properties of spatial vision (e.g., contrast sensitivity and its dynamics) that, at present, are solely attributed to neural mechanisms. Under- standing the functional implications of fixational instability may also lead to new treatment approaches for visual impairments in the many disorders whose manifestations include abnormal fixational eye movements.